STRUCTURAL CHARACTERIZATION OF 2 MODIFICATIONS OF TRIS(TETRAHYDROFURAN)(TRIS(TRIMETHYLSILYL)SILYL)LITHIUM - A COMPOUND WITH A SI-29-LI-7 NMR COUPLING

The crystal structures of (Me3Si)4Si.THF (a = b = 12.310(1) angstrom, c = 17.308(2) angstrom, tetragonal, space group P4/ncc (No. 130), Z = 4), [(Me3Si)4Si] [(Me3Si)3SiLi(THF)3]2 (a = b = 15.146(8) angstrom, c = 21.338(10) angstrom, trigonal, space group P3clBAR (No. 165), Z = 2), (Me3Si)3SiLi(THF)3 (a = b = 31.342(4) angstrom, c = 22.000(6) angstrom, rhombohedral, space group R3c (No. 161), Z = 24), and (Me3Si)3SiSi(SiMe3)3.THF (a = 11.360(3) angstrom, b = 17.131(6) angstrom, c = 18.952(6) angstrom, beta = 91.21(3)-degrees, monoclinic, space group P2(1)/c (No. 14), Z = 4), have been determined by low-temperature X-ray analysis. Whereas the small Si-Si-Si angle in (tris(trimethylsilyl)silyl)lithium of 102.1-degrees supports the ionic character of the Si-Li bond, there must be at least some covalent contribution leading to a (1J[Si,Li])-Si-29-Li-7 coupling constant of 38.6 Hz. A modified preparation for (Me3Si)3SiLi(THF)3 is reported giving the pure product rather than the cocrystallization product [(Me3Si)4Si] [(Me3Si)3SiLi(THF)3]2, Where the starting material is still present in the ratio 1:2. The pure product is far more reactive than the cocrystallized material and is as pyrophoric as (t)BuLi powder.